The most widely used prior art pigments consist of particles which absorb a portion of the incoming light and reflect the remainder. The reflected portion of the light determines the color impression.
In addition there are pigments whose color is due to interference effects. These pigments are known as pearl luster pigments. They consist of a base material, most frequently mica platelets, to which thin layers of substances having different optical densities have been applied. These layers often consist of heavy metal compounds (Ullmanns Encyclopadie der technischen Chemie, 4th edition, 1976, Volume 18, p. 631-634). The application of pearl luster pigments containing heavy metals is problematic for reasons of environmental pollution during processing and disposal.
EP-A 383,376 describes the use of cholesteric liquid crystals instead of heavy metal compounds for coating mica platelets and other organic and inorganic base materials. While it is true that the use of cholesteric liquid crystals avoids the disadvantage of coatings containing heavy metals, the base materials themselves often contain heavy metals. Moreover, the above mentioned pigments must be able to meet processing and application parameters. For example, the base material must not exceed a particular thickness otherwise the preparation of finely divided pigments is hampered. It is true that the base materials are very thin in the case where mica or metal flakes are used, but they need to be coated on the front and back in layer thicknesses to be strictly maintained.